Double acting reciprocating piston pumps are an efficient means for flowing liquids because the pump both delivers and aspirates the liquid to be pumped with each stroke of the piston. Many United States Patents have been issued for improvements to double acting pumps. For example: U.S. Pat. No. 679,454 issued to Conner on July 30, 1901 disclosed air chambers to dampen pressure pulses of a manually operated double acting pump; and U.S. Pat. No. 1,880,494 issued to Sandage on Oct. 4, 1932 disclosed a double acting pump, wherein the piston was directly reciprocated by an eccentric cam.
Most piston pumps such as the pump of the Sandage patent, deliver the liquid to be pumped at a varying rate during each stroke of the piston. This varying rate results in pressure pulsations in the liquid being pumped. Some pumping applications are best made with a substantially pulseless pump, e.g., the eluent pump of a liquid chromatography chemical analysis system. Piston pumps have been developed that are substantially pulseless, e.g., the two piston cam driven pump disclosed in U.S. Pat. No. 4,028,018 issued on June 7, 1977 to Audsley.
Rotary cams are generally classified as uniform motion cams and uniformly accelerated motion cams. E. Oberg & F. Jones, Machinery's Handbook, 712 (1974). The uniform motion cam rotated at constant angular velocity moves the cam follower at substantially the same velocity from the beginning to the end of each stroke of the cam follower. Uniform motion cams are usually heart shaped. Uniform motion cams impart relatively sudden changes of direction to the cam follower at the beginning and the end of each stroke of the follower. This characteristic is substantially eliminated by the use of a uniformly accelerated motion cam of which an eccentric cam is an example. The cam 13 of FIG. 2 of the Audsley patent is an example of an asymmetrical hybrid four zone cam having two zones of uniformly accelerated motion and two zones of constant motion (see column 2, lines 63-68 of the Audsley patent).
It would be an advance in the art of double acting pumps if such a pump could be developed that was substantially pulseless and that delivered the liquid to be pumped at substantially the same flow rate during each entire stroke of the pump's piston.